1. Field of the Invention
The invention relates generally to the field of drill bits used to bore holes through earth formations. More particularly, the invention relates to methods and structures for improving the durability of roller cone drill bits.
2. Background Art
Drill bits used to bore drill holes or wellbores through earth formations include roller cone bits. Typical roller cone bits include a bit body made from steel or similar material. The bit body includes one or more, and typically three, legs which depend from the bit body. The bit body is usually adapted to be threadedly or otherwise coupled to a drilling tool assembly (“drill string”) which rotates the bit body during drilling. The legs include a bearing journal, onto each of which is rotatably mounted a roller cone. The roller cone includes a plurality of cutting elements disposed at selected positions about the surface of the cone. The cutting elements may be hard metal or composite inserts, milled steel teeth, or any combination thereof depending on the type of earth formation that is expected to be drilled with the particular drill bit.
In many types of roller cone bits, the roller cone is sealed with respect to the bearing journal to exclude fluids and debris from the wellbore from entering the bearing journal. The seal element is often an elastomer ring or similar device, while a lubricant filling the bearing surfaces on the journal is typically some form of petroleum based grease or the like. An exterior, exposed face of the bearing journal, outside of the volume sealed by the roller cone seal, is formed in various ways known in the art so as to maximize exclusion of cuttings and debris from the seal area. This exposed face is typically shaped so substantially conform to the curvature of the inside (bearing) surface of the roller cone, and is known in the art as a “shirttail” portion of the bit leg.
Typically the roller cones have sizes, and cutting elements arranged thereon, to substantially avoid contact between the wellbore wall and the shirttail portion of the leg. Further, the shirttail portion is itself shaped to minimize such contact during drilling of earth formations. In certain circumstances, such contact is difficult to avoid. Typical roller cone drill bits also include therein fluid discharge nozzles (“jets”), which provide a path for discharge of drilling fluid from the interior of the drilling tool assembly to cool, lubricate and clean the roller cones, and to lift formation cuttings out of the wellbore as the wellbore is being drilled. Often, such drilling fluid is circulated through the wellbore at high rates to enable adequate lifting of drill cuttings. In certain drilling operations, such as with drill strings which include steerable mud motors and the like, it has been observed that the shirttail portion of typical prior art roller cone drill bits is subject to high rates of erosion due to fluid flow past the shirttail, in addition to any abrasive wear which sometimes may result from the previously described wall contact.
Techniques known in the art for reducing wear on bit structures include attachment of hardface and/or superhard material inserts or similar structures into the wear prone areas. These type of wear resistance structures are not particularly effective in reducing wear caused by erosion because they only serve to prevent contact between surfaces. Techniques known in the art for reducing bit structure wear also include thermally applied hardfacing. Typical hardfacing thermal application techniques tend to raise the temperature of the applied-to structure so a degree which makes the use of such techniques impracticable for roller cone drill bits because of possible damage to the seals and lubricant, at least. Such techniques when used prior to assembly of the roller cones to the leg may also result in some changes to the fracture toughness of the leg material, and have therefore not been widely used. In many cases, erosion on the shirttail is not a problem, meaning that erosion damage to the shirttail occurs at such slow rates relative to wear of the bearing structure and cutting elements on the roller cones, as to make hardface application to the shirttail on all drill bits uneconomical.
It is desirable to have a technique for reducing wear on the shirttail portion of a roller cone drill bit which can be selectively applied to already assembled bits, and which minimizes possible damage to bit structures by its application.